convert_to_fes: fix doc

[makedoc]

src/gridtools/ConvertToFES.cpp

@@ -32,7 +32,7 @@ This action allows you to take a free energy surface that was calculated using t
 action and to convert it to a free energy surface.  This transformation performed by doing:
 
 \f[
-F(x) = -k_B T \ln H(x)\f$
+F(x) = -k_B T \ln H(x)
 \f]
 
 The free energy calculated on a grid is output by this action and can be printed using \ref DUMPGRID